During the recovery of hydrocarbons from subterranean formations, significant amounts of displaceable hydrocarbons are left behind because the reservoir strata are non-homogeneous. The natural drive fluids (e.g. brine or gaseous hydrocarbons) or secondary recovery flooding fluids (e.g. brine, steam, or carbon dioxide) flow through the more permeable strata of the reservoir resulting in progressively less hydrocarbon being recovered per unit volume of fluid produced. This increased ratio of drive or flooding fluid to hydrocarbon is usally referred to as early breakthrough of flooding fluid from injector wells or as excessive water encroachment in producer wells. It is desirable to increase the sweep efficiency, thereby producing more hydrocarbon per unit volume of fluid recovered. Chemicals have been used to achieve increased sweep efficiencies and to enhance hydrocarbon recovery by modifying the permeability of reservoir strata so that the natural or flooding fluids flow through hydrocarbon-rich strata in preference to strata that are more permeable and contain none or smaller amounts of recoverable hydrocarbons.
The chemicals that can be used to modify the permeabilities of subterranean reservoirs must be easily pumpable (i.e. not excessively viscous) so that they can be easily emplaced into the reservoir sufficiently far from the wellbore so as to be effective. It is desirable for the chemicals to reduce the permeability of the reservoir to the driving or flooding fluids while retaining most hydrocarbon permeability. Their emplacement should be selective in that they are emplaced into and reduce the permeability of reservoir strata more permeable to the driving fluids without seriously affecting the less permeable strata. Gels formed by polyacrylamide (U.S. Pat. No. 3,490,533) or polysaccharides with cations (U.S. Pat. Nos. 3,581,524; 3,908,760; 4,048,079) have been used as permeability modifiers for subterranean reservoirs. However, their application has been limited to subterranean reservoirs with ambient temperatures of less than about 70.degree. C. Major difficulties encountered in using gel forming chemicals in high temperature reservoirs (i.e.&gt;70.degree. C.) include:
(1) Increasing gelation rate with increasing temperature resulting in premature gel formation and therefore plugging of the reservoir strata nearest the wellbore with the needed permeability modification at substantial distances from the wellbore being unattainable.
(2) Over-crosslinking and syneresis of the gel at elevated temperatures and in high total dissolved solid (TDS) reservoir brines which reduce the effectivensss of the gel as a permeability modifier.
(3) Decomposition by oxidation and/or hydrolytic mechanisms of polysaccharides and polyacrylamides at elevated temperatures with a net result of destruction of gel character. Some of the acrylamide groups of the polyacrylamide hydrolyze to carboxylic acid groups at elevated temperatures which causes the polyacrylamide to form calcium and magnesium salts resulting in precipitates which are undesirable.
Melamine resins have been employed for a number of uses including adhesives, laminating resins, molding compounds, coatings, textile finishes and paper treatment (I. H. Updegraff et al, Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 2, pp. 440-469, 1982). The subject of this invention, sulfomethylated melamine polymers, has also found a number of uses. U.S. Pat. No. 2,407,599 claims a process for preparing thermosetting resinous products containing sulfonate groups by reacting at pH 4-10;(1) an aldehyde (e.g. formaldehyde, (b) a water soluble salt of sulfurous acid, and (c) a member of the class consisting of carboamides, polyamino diazines and polyamino triazines (e.g. melamine). The aldehyde is present in an amount between about 0.5 and 2.0 moles per reactive amino group and said salt in an amount between about 0.05 to about 0.4 mole of bisulfite per mole of aldehyde. This patent also claims compositions prepared by the claimed process. U.S. Pat. No. 2,562,866 claims a process for preparing melamine-aldehyde sealing agents for porous formations penetrated by boreholes as well as compositions prepared by the process. These compositions are not water soluble and must be used as an aqueous suspension. A process for preparing a solid thermosetting resin useful as a textile finishing agent and a paperwet strength agent and compositions prepared by the process are claimed by U.S. Pat. No. 2,603,623. The process consists of reacting a water soluble preformed etherified melamine-formaldehyde resin in an aqueous solution with an alkali metal salt of sulfurous acid sufficient to provide at least 0.1 mole of combined sulfur trioxide for each mole of melamine. Sulfomethylated melamine resins are claimed as water loss additives for drilling fluids in U.S. Pat No. 2,730,497 with specific claims being for drilling fluid compositions. U.S. Pat No. 2,730,516 claims a process for producing non-thermosetting, permanently water-soluble melamine-formaldehyde resinous compositions modified with an alkali metal salt of sulfurous acid which are useful as replacements for natural hydrophilic hydrocolloids (e.g. gelatin and agar). The mole ratios of reactants are 1.0 melamine, 1.75-6.0 formaldehyde and 0.75-2.0 of sulfurous acid salt. A range of pH of between about 1.5 and about 3.7 is necessary during the polymerization of the product, and the temperature range is 40.degree.-55.degree. C. Compositions prepared by this process are also claimed.
U.S. Pat No. 4,473,119 claims a process for selectively plugging highly permeable zones in a subterranean formation by injecting a melamine-formaldehyde solution via a well into the highly permeable zones. A solution of melamine and formaldehyde in a water soluble solvent (e.g. alcohol) is injected into an alkaline formation environment. The solution preferentially migrates into the highly permeable zones which contain substantial amounts of water. The solution reacts in situ at the formation temperature to produce a nonflowing, water insoluble resin. The resin permanently plugs the highly permeable zones. Although the method of this patent appears simple to operate, those skilled in the art will recognize several deficiencies in this process. (1) The methylolmelamine prepared by the reaction of melamine and formaldehyde in an aqueous solution is not soluble in brines of &gt;1.0% TDS. The presence of alcohols in a brine does not increase the solubility of the methylolmelamine in that brine. Often in an oil field sources of large amounts of fresh water (i.e. &lt;1% TDS) are not available or are available at great expense. Furthermore, in brines of &gt;1% TDS the resins formed from methylolmelamine at high temperatures (i.e.,&gt;70.degree. C) are precipitate-like in nature and useless as plugging agents. Thus, it is highly desirable to have gel-forming compositions that are soluble in the reservoir brine (e.g. brines up to about 20% TDS) and then to form high temperature stable gels in these brines in the reservoir.
(2) An alternate embodiment describes the use of a slurry of melamine and formaldehyde in water which is then injected immediately into the formation. This technique has the potential for severe face plugging of the formation at the wellbore. Thus, aqueous solutions with no suspended particles are highly desirably for proper emplacement of the gel-forming composition.
(3) It is highly desirable to have a gel-forming composition that will selectively enter the strata of the reservoir more permeable to driving fluid and form gels; thus, effecting a selective permeability modification of more permeable strata of the reservoir and leaving the less permeable strata relatively less affected. In addition, it is desirable that the porous medium, where the gelled gel-forming compositions are emplaced, be selectively more permeable to hydrocarbons than to flooding fluids. In general, insoluble resins such as those exemplified in U.S. Pat. No.4,473,119, form undesirable plugs that block completely all flow through the treated strata of the reservoir including the flow of the desired hydrocarbons. They are also nearly impossible to remove from the reservoir if improperly emplaced due to their insolubility.
Thus, it can be seen that there is a need for brine compatible, high temperature stable and high temperature emplacable permeability modifiers for use in subterranean hydrocarbon-bearing reservoirs that do not completely block the flow of hydrocarbons in the affected strata.